1. Field of the Invention
This invention relates to an image forming apparatus, and more specifically to an electrostatic type copying machine having a photoconductor, and designed to copy an optical pattern image which is the information formed on an original, by forming on the photoconductor a latent image corresponding to the image, rendering the latent image visible, and electrostatically transferring the visible image from the photoconductor to a recording material.
2. Description of the Related Art
Most electrostatic type copying machines include a photoconductor, a developing mechanism, a material delivering mechanism, and a cleaning unit. A latent image of the optical pattern image of the object, e.g., the information printed on a document to be copied, is formed on the photoconductor. The developing mechanism supplies developer (generally known as "toner") to the photoconductor, thereby developing the latent image into a visible one. The material delivering mechanism supplies the recording material, such as a sheet of paper. The image is electrostatically transferred from the photoconductor to a recording material, to make a hard copy. The cleaning unit removes the residual developer from the photoconductor.
The electrostatic type copying machine further includes an illuminating device. This device applies light to the document, and the light is reflected from the document. The light reflected from the document defines an optical pattern corresponding to the image formed on the document. The optical pattern is supplied to the photoconductor, whereby a latent image is formed on the photoconductor. As has been described, the developing mechanism supplies developer to the photoconductor, thereby developing the latent image into a visible one. The visible image is put on recording material, whereby a hard copy of the image information is produced.
The optical density of the image, thus copied, depends on the intensity of the light which the illumination device applies to the document. In order to produce a hard copy which is as clear as possible, the intensity of the light is adjusted in accordance with two items of information. The first item is the optical density of the image formed on the document. The second item is the optical density of the document, i.e. background of the document (hereinafter referred to as "base density").
To copy the image having a low optical density, such as an image formed by a pencil on a document, a light beam having a relatively low intensity is applied to the document from the illuminating device. On the other hand, to copy the image formed on a document having a high base density, such as a page of newspaper, a light beam having a relatively high intensity is applied to the document from the illuminating device.
The base density of a document and the average optical density of the image formed on the document are detected before the copying operation is started. These densities are used to determine the best possible intensity for the light to be applied to the document. In other words, the intensity of the light is automatically adjusted to an optimal value in accordance with the base density of the document and the average optical density of the image formed on the document.
Thanks to the intensity of the document-illuminating light, thus adjusted, conventional copying machines can, indeed, produce a clearer copy of the image formed on a document. However, they cannot produce clear copies when the image formed on a document consists of various components, such as photographs, characters, and graphics, which greatly differ in their average optical densities. If the intensity of the document-illuminating light is adjusted to the base density of the document and also to the average optical density of one image component, another image component having a far lower density, such as a pencil sketch, will not be copied at all in the worst case. On the other hand, if the intensity of the light is adjusted to the average optical density of an image component of a low density, such as a pencil sketch drawn on a news paper page, the ground of the image copied will be too dark.